1. Field of the Invention
The present invention relates to a current-controlled motor drive circuit for implementing, in a direct drive system, motor characteristics including high output, high efficiency, and minimized variations in revolution in an extremely low speed revolution range, e.g. 50 rpm to 100 rpm required of, for example, a drum drive motor of a copier or a main motor of a laser beam printer.
2. Description of the Related Art
Hitherto, drum driving motors of copiers or main motors of laser beam printers are generally controlled by pulse width modulation (PWM) using an H bridge. A motor drive circuit using the H bridge employs a system in which only a source side or a sink side is switched in an H-bridge switching mode for conducting the PWM control. In this system, an attempt to carry out sinusoidal current control (wherein sinusoidal information is added to a current command value to provide faithful sinusoidal waves while detecting motor coil current) as indicated by the solid line in FIG. 1 develops a phenomenon in which a current fall delays as indicated by the dashed line in the graph. This causes a sudden drop in current when an energizing direction is switched. Such sudden change in motor coil current in the vicinity of zero cross at which the current direction is switched is not negligible because it worsens variations in revolution. Furthermore, the motor coil current deviates from a sinusoidal command value at the time of current switching or commutation, making it impossible to obtain a current waveform that is faithful to the command value. This happens because of the following reason. When only a source side shown in FIG. 2A or a sink side shown in FIG. 2B is switched, in either case, a regenerative current i produced by counter-electromotive force passes in an independent closed loop via a flywheel diode D2 at the time of regeneration when either is switched OFF. At this time, a voltage V of a motor coil 7 is clamped by the forward voltage of a diode D2 and a FET saturation voltage (and a voltage drop in a shunt resistor Rs is added in the case of switching of the source side), so that the reduction in the regenerative current i becomes slow or is delayed. In the circuits shown in FIGS. 2A and 2B, reference characters Q1, Q2, Q3, and Q4 denote field effect transistors (FETs).
FIG. 3 shows a conventional H-bridge type current detecting circuit. In the drawing, when the source side and the sink side are switched simultaneously, there will be no problem in a powering mode because all motor coil current passes through the shunt resistor Rs. In a regeneration mode (at the moment of switching OFF), however, the regenerative current i from a counter-electromotive force flows over two paths (P1) and (P2) as shown in the drawing. In the case of path (P1), the current detecting circuit determines that no regenerative current is flowing whereas the regenerative current i is actually passing through the motor coil 7 because the current does not pass through the shunt resistor Rs in path (P1). In the case of path (P2), the current detecting circuit determines that the regenerative current is flowing in the reverse direction whereas the regenerative current i is actually flowing through the motor coil 7 in the same direction as that immediately before current supply was cut off because the direction of current passing through the shunt resistor Rs is reversed between the powering mode and the regenerative mode in the case of path (P2). Thus, the conventional current detection system has been posing a problem in that the current in the motor coil cannot be accurately detected.
There has been another problem. The regenerative mode means a switching OFF mode, providing a time zone or range wherein no control can be conducted. As a matter of fact, a PWM switching waveform directly appears or overlaps in the current detecting circuit of FIG. 3, inversely affecting the current control of the motor coil 7.
As set forth above, the conventional current detecting system has been presenting serious problems in both a drive mode of a logic circuit for driving a switching device of an H-bridge and a current detection method employing a shunt resistor, which results in making impossible to detect motor coil current quickly and accurately. As a result, the conventional current detecting system has a shortcoming in which it cannot control motor coil current in strict accordance with a sinusoidal current command value.